Image sensor

ABSTRACT

An image sensor includes an array of readout circuits in non-organic technology and photodiodes made of organic materials.

The present patent application is a continuation of U.S. patentapplication Ser. No. 16/956,515, which is a 371 U.S. national stagepatent application of PCT/FR2018/052894, which claims the prioritybenefit of French patent application 17/62783, the disclosures of whichare herein incorporated by reference.

BACKGROUND

The present disclosure relates to an image sensor or electronic imager.The present disclosure more particularly relates to devices comprisingan organic photodetector.

DISCUSSION OF THE RELATED ART

Image sensors are used in many fields, particularly in electronicdevices due to their miniaturization. Image sensors can be found inman-machine interface applications as well as in image captureapplications.

The present disclosure more particularly relates to image sensors formedfrom a photodetector made of organic materials.

An example of a photodetector made of organic materials is described indocument U.S. Pat. No. 9,417,731.

Document US 2015/060775 describes an organic photodiode with a doubleelectron blocking layer.

Document US 2009/090903 describes a CMOS image sensor having thiophenederivatives.

Document WO 99/39372 describes image sensors formed from organicsemiconductors.

SUMMARY

An embodiment overcomes all or part of the disadvantages of imagesensors integrating a photodetector made of organic materials.

An embodiment provides a solution avoiding the use of organictechnologies for the entire image sensor.

Thus, an embodiment provides an image sensor comprising an array ofreadout circuits in non-organic technology and photodiodes made oforganic materials.

According to an embodiment, the readout circuits are in CMOS technology.

According to an embodiment, each photodiode is separated from the nextphotodiodes by spacers made of an electrically-insulating material.

According to an embodiment, the spacers are made of a transparentmaterial.

According to an embodiment, the spacers are opaque to light.

According to an embodiment, the spacers absorb light.

According to an embodiment, each photodiode comprises a first electrodehaving the size of a pixel or sub-pixel of the associated readoutcircuit.

According to an embodiment, the first electrode is connected to the gateof a first transistor of the readout circuit.

According to an embodiment, the first electrode is coupled to a gate ofa first transistor of the readout circuit via one or a plurality ofadditional transistors.

According to an embodiment, a second electrode of the photodiodes iscommon to a plurality of readout circuits.

According to an embodiment, the second electrode is formed of atransparent organic conductive layer.

According to an embodiment, each pixel or sub-pixel is inscribed withina square having an approximately 50-μm side length.

According to an embodiment, the pixels or sub-pixels are laterallyspaced apart by less than 5 μm.

According to an embodiment, the second electrode is covered with atranslucent protection layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages, as well as others, will bedescribed in detail in the following description of specific embodimentsgiven by way of illustration and not limitation with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view schematically showing an embodiment of animage sensor;

FIG. 2 is a cross-section view of an embodiment of an image sensor;

FIG. 3 is a simplified top view of the image sensor of FIG. 2 ;

FIG. 4 shows the simplified electric diagram of a CMOS readout circuitwith three transistors; and

FIG. 5 very schematically shows an example of layers of an organicphotodiode.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings.

For clarity, only those steps and elements which are useful to theunderstanding of the embodiments which will be described have been shownand will be detailed. In particular, the operation of the image sensorhas not been detailed, the described embodiments being compatible withusual sensors, be they pure image sensors or sensors integrating adisplay. Further, the other components of the electronic deviceintegrating the image sensor have not been detailed either, thedescribed embodiments being compatible with the other usual componentsof electronic devices with an image sensor application.

Unless indicated otherwise, when reference is made to two elementsconnected together, this signifies a direct connection without anyintermediate elements other than conductors, and when reference is madeto two elements coupled together, this signifies that these two elementscan be connected or they can be coupled via one or more other elements.

In the following description, when reference is made to the terms“approximately”, “about”, and “in the order of”, this means within 10%,preferably within 5%.

Further, in the following description, when reference is made to termsqualifying absolute positions, such as terms “high”, “low”, “left”,“right”, etc., or relative positions, such a terms “above”, “under”,“upper”, “lower”, etc., or to terms qualifying orientation, such as term“horizontal”, “vertical”, unless otherwise specified, reference is madeto the orientation of the drawings.

FIG. 1 is a perspective view schematically showing an example of animage sensor 10.

Sensor 10 comprises an assembly of photon sensors, or photodetectors 12,for example, a photodetector array. In this example, photodetectors 12are arranged on a planar surface. Embodiments may however be providedwhere photodetectors 12 are arranged on a non-planar surface.Photodetector array 12 may be topped with a transparent or translucentprotection coating 14, for example, a glass plate, a plastic coating, adeposit by using a technology of successive depositions of atomicmonolayers or a deposition of one or a plurality of other passivationlayers. Photodetector array 12 is associated with an array of readoutcircuits 16, for measuring the signals captured by the photodetectors.Readout circuit means an assembly of transistors for reading out,addressing, and controlling the pixel or sub-pixel defined by thecorresponding photodetector. Such a circuit may be integrated on thesame silicon surface as the pixel array or network, or be a circuit oran assembly of electronic circuits associated with the pixel array.

According to the photodetectors used in array 12 and to the possiblepresence of filters, device 10 is capable of capturing shadows, infraredimages, black and white images, color images (RGB), etc.

Image sensor 10 may further be associated with a display, to form aninteractive interface.

The embodiments described hereafter apply to all these applications,including combined.

Organic sensors are generally arranged with first electrodes formingstrips along a first direction, second electrodes forming strips along asecond direction, and with the active material therebetween. Each pixelof the sensor comprises a photodetector element made of organicmaterials at the intersection between the electrode strips. The circuitsfor interpreting the signals across the electrodes are generallyexternal to the plane of the pixel array.

FIG. 2 is a partial cross-section view of an embodiment of an imagesensor 10.

FIG. 3 is a simplified top view of the image sensor of FIG. 2 , at thelevel of organic elements 10.

According to the described embodiments, it is provided to form an imagesensor by associating photodetectors or organic photodiodes OPD andnon-organic readout circuits, in CMOS technology on a semiconductorsubstrate. More particularly, it is provided to stack an array oforganic photodiodes OPD on an array of CMOS readout circuits, or CMOSpixels, each photodiode forming the detection element of a pixel 21 orof a sub-pixel of the image sensor.

All that will be described hereafter in relation with a pixel moregenerally applies to a sub-pixel, in particular for a color image sensorhaving each pixel generally formed of four sub-pixels.

As illustrated in FIGS. 2 and 3 , each photodiode is separated from theneighboring photodiodes by a spacer 23. Thus, spacers 23 delimit theactive areas of photodiodes to electrically insulate them from oneanother. Spacers 23 are thus made of an electrically-insulatingmaterial.

Further, at least one of the electrode layers (typically cathode layer25) is structured (pixelated) to define a cathode of a photodiodevertically in line with a CMOS circuit 27 of the corresponding pixel.The other electrode (anode 29) may also be pixelated, but may as avariant form strips. In the case where the anode is pixelated, the anodeelectrodes are however interconnected at least in a first direction toallow their biasing.

The CMOS circuits or pixels are very schematically shown, theirmanufacturing being usual and the described embodiments being compatiblewith usual CMOS circuits, formed on a silicon substrate or other, basedon non-organic, semiconductor, and conductive materials. The metalinterconnection levels 31 of the transistors have been symbolized inFIG. 2 .

According to an embodiment, spacers 23 are made of a transparentmaterial.

According to another embodiment, spacers 23 are preferably made of anopaque and/or light-absorbing material. This enables to take advantageof the granularity of a CMOS imager having its pixels close to oneanother, while avoiding light leakages from one pixel to the next pixel.Indeed, the pixels of a CMOS sensor on silicon have, as an example, anapproximately 50-μm side length and are distance from one another byless than 5 μm. Now, the thickness of the active layers of an organicphotodiode is generally a few μm and is thus of the same order ofmagnitude as the interval between two pixels. Making the spacers opaqueor absorbing avoids light interferences between pixels and thus improvesthe definition of the image sensor.

As a specific embodiment, spacers 23 are made of resin, offluorine-based resin or on resin based on polyimide or parylene or otherinsulating materials.

FIG. 4 shows the simplified electric diagram of a CMOS readout circuitor pixel 27 with three transistors, associated with an organicphotodiode OPD.

Readout circuit 27 comprises a MOS detection transistor 41, in serieswith a MOS selection transistor 43, between two terminals 45 and 47 ofapplication of a DC voltage VDD. The gate of transistor 43 is intendedto receive a pixel selection signal SEL. The gate 49 of transistor 41defines an input terminal of the CMOS circuit. Gate 49 is coupled, by aMOS reset transistor 51, to a terminal of application of a resetpotential VR. The gate of transistor 51 is intended to receive a pixelreset control signal R (for discharging the gate 49 of detectiontransistor 41).

The gate 49 of transistor 41 is coupled to the cathode electrode 25 ofthe organic photodiode OPD of the considered pixel. The anode electrode29 of photodiode OPD is coupled to a fixed potential (not shown) or toground, but may be on order taken to different potentials.

The operation of a CMOS readout circuit is known and is not modified bythe described embodiments. Further, CMOS readout transistors with fourtransistors (4T) where a transfer transistor is interposed between thecathode 25 of photodiode OPD and the gate 49 of detection transistor 41,or even CMOS readout transistors with more than four transistors with aplurality of additional transistors between cathode 25 and the gate oftransistor 49, may be used.

FIG. 5 very schematically illustrates an example of layers of an organicphotodiode.

In this example, each organic photodiode OPD comprises a stack, in thefollowing order from a cathode electrode 25 connected to the CMOScircuit:

a layer 61 made of a heavily-doped organic semiconductor polymer (holedonor layer);

a layer 63 made of an organic semiconductor polymer, for examplepoly(3-hexylthiophene) or poly(3-hexylthiophene-2,5-diyl) (P-typesemiconductor), known as P3HT, or [6,6]-phenyl-C₆₁-methyl butanoate(N-type semiconductor), known as PCBM;

a layer 65 of heavily-doped transparent organic semiconductor polymer(electron donor layer), for example, a polymer known as Pedot-PSS, whichis a mixture of poly(3,4)-ethylenedioxythiophene and of sodiumpolystyrene sulfonate; and a transparent layer 67, for example, made ofindium-tin oxide (ITO), of Pedot-PSS, or of a transparent conductiveorganic material, for example, based on conductive nanowires, ongraphene, or on conductive or semiconductor nanoparticles.

Cathode electrode 25 is for example made of aluminum, of silver, of ITO,of molybdenum tantalum (MoTa), of tungsten, of chromium, of Pedot-PSS,or another metal or conductive or semiconductor organic material.

Layers 61 and 65 have the function of adapting the work function betweenactive area 63, similar to a PN junction, and the cathode and anodeelectrodes, respectively.

Accordingly, according to the work function of the different layers,layer 67 or layer 65 may directly form anode 29.

Similarly, according to the work function of layer 25, layer 61 may beomitted.

Organic photodiode manufacturing techniques are described, for example,in document U.S. Pat. No. 9,417,731.

An advantage of the described embodiments is that they make it possibleto combine technologies of manufacturing of CMOS semiconductor readoutcircuits on silicon or another non-organic semiconductor substrate, andorganic photodiode manufacturing technologies.

Another advantage is that makes it possible to form an infrared (around1,400 nm) or near infrared (around 940 nm) sensor, taking into accountthe admissible wavelengths for the organic photodiodes and theresolution of the non-organic CMOS sensors.

Another advantage of the described embodiments is that the performedpixelization is compatible with the addition of microlenses on the anodeside.

Among the applications of an image sensor or imager such as described,gaze tracking devices, facial recognition, fingerprint sensors, nightvision devices, cameras, etc. should be noted.

Various embodiments have been described. Various modifications willoccur to those skilled in the art. In particular, the selection of thematerials forming layers 61, 63, and 65 depends on the application andparticularly on the wavelengths which are desired to be detected.Further, a reflective material may be provided for cathode 25 to improvethe sensitivity of the photodiodes. Gold or an alloy of tungsten andmolybdenim may in particular be provided. Further, the practicalimplementation of the embodiments which have been described is withinthe abilities of those skilled in the art based on the functionalindications given hereabove.

1. An image sensor comprising an array of readout circuits innon-organic technology and photodiodes made of organic materials.
 2. Thesensor according to claim 1, wherein the readout circuits are made inCMOS technology.
 3. The sensor according to claim 1, wherein eachphotodiode is separated from the next photodiodes by spacers made of anelectrically-insulating material.
 4. The sensor according to claim 3,wherein the spacers are made of a transparent material.
 5. The sensoraccording to claim 3, wherein the spacers are opaque to light.
 6. Thesensor according to claim 3, wherein the spacers absorb light.
 7. Thesensor according to claim 1, wherein each photodiode comprises a firstelectrode having the size of a pixel or sub-pixel of the associatedreadout circuit.
 8. The sensor according to claim 7, wherein the firstelectrode is connected to a gate of a first transistor of the readoutcircuit.
 9. The sensor according to claim 7, wherein the first electrodeis coupled to a gate of a first transistor of the readout circuit viaone or a plurality of additional transistors.
 10. The sensor accordingto claim 1, wherein an electrode of the photodiodes is common to aplurality of readout circuits.
 11. The sensor according to claim 10,wherein the electrode is covered with a translucent protection layer.12. The sensor according to claim 10, wherein the electrode is formed ofa transparent organic conductive layer.
 13. The sensor according toclaim 7, wherein each pixel or sub-pixel is inscribed within a squarehaving a side length smaller than 50 μm.
 14. The sensor according toclaim 7, wherein the pixels or sub-pixels are laterally spaced apart byless than 5 μm.